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If n is even, then in the expansion of open parentheses 1 plus fraction numerator x to the power of 2 end exponent over denominator 2 factorial end fraction plus fraction numerator x to the power of 4 end exponent over denominator 4 factorial end fraction plus midline horizontal ellipsis close parentheses to the power of 2 end exponent, the coefficient of x to the power of n end exponent text  is end text

  1. fraction numerator 2 to the power of n end exponent over denominator n factorial end fraction    
  2. fraction numerator 2 to the power of n end exponent minus 2 over denominator n factorial end fraction    
  3. fraction numerator 2 to the power of n minus 1 end exponent minus 1 over denominator n factorial end fraction    
  4. fraction numerator 2 to the power of n minus 1 end exponent over denominator n factorial end fraction    

The correct answer is: fraction numerator 2 to the power of n minus 1 end exponent over denominator n factorial end fraction


    open parentheses 1 plus fraction numerator x to the power of 2 end exponent over denominator 2 factorial end fraction plus fraction numerator x to the power of 4 end exponent over denominator 4 factorial end fraction plus midline horizontal ellipsis close parentheses to the power of 2 end exponent equals open parentheses fraction numerator e to the power of x end exponent plus e to the power of negative x end exponent over denominator 2 end fraction close parentheses to the power of 2 end exponent equals fraction numerator 1 over denominator 4 end fraction open parentheses e to the power of 2 x end exponent plus e to the power of negative 2 x end exponent plus 2 close parentheses
    equals fraction numerator 1 over denominator 4 end fraction open curly brackets 2 open parentheses 1 plus fraction numerator left parenthesis 2 x right parenthesis to the power of 2 end exponent over denominator 2 factorial end fraction plus fraction numerator left parenthesis 2 x right parenthesis to the power of 4 end exponent over denominator 4 factorial end fraction plus midline horizontal ellipsis close parentheses plus 2 close curly brackets
    text Therefore, the coefficient of  end text x to the power of n end exponent left parenthesis n text  even  end text right parenthesis
    fraction numerator 1 over denominator 2 end fraction open curly brackets fraction numerator 2 to the power of n end exponent over denominator n factorial end fraction close curly brackets equals fraction numerator 2 to the power of n minus 1 end exponent over denominator n factorial end fraction

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